1. Field of the Invention
This invention relates generally to an optical device employing an optical thresholder and, more particularly, to an optical comparator that employs an optical thresholding device positioned in a propagation path of an optical input beam that provides an indicator signal indicative of whether the intensity of the input beam is above a threshold.
2. Discussion of the Related Art
Advances in signal processing technology, including the need for greater processing speeds, increased channel bandwidths and improved transmission reliability, has resulted in a steadily growing focus on the optical domain and the vast potential that lies therein with respect to these parameters. However, optical technology, as compared to electrical and radio frequency based technology, lacks the necessary technical sophistication in many areas. Particularly, the desire for high-speed, large-bandwidth processing devices employing digital optics has been hampered by the lack of many basic optical devices and technologies readily available in the RF domain.
Optical comparators comprise an example of such a device that has not heretofore met basic design requirements. Similarly, variable optical signal attenuator is another example of a device that has not heretofore met basic attenuator design requirements. An optical comparator that compares an optical signal with an optical threshold and indicates whether the threshold has been exceeded and/or an optical signal attenuator that decreases the intensity of an optical signal by a predetermined amount with little or no reliance on RF technology would reduce system complexity and provide for high speed and large bandwidth processing advances.
What is needed is an efficient, cost effective and reliable optical comparator and variable optical signal attenuator that has application to be used in an optical signal processing system. It is therefore an object of the present invention to provide such an optical comparator and an optical signal attenuator.
In accordance with the teachings of the present invention, two optical devices are disclosed that employ an optical thresholding device positioned in the propagation path of an optical input beam.
For the optical comparator the optical thresholding device saturates and turns transparent if the intensity of the optical beam that impinges it is above a predetermined threshold level which is designed into the device. If the input beam to the optical comparator saturates the optical thresholder it outputs an indicator signal identifying the saturation and that the threshold has been exceeded. The presence of an indicator signal gives an indication of the intensity of the input beam. In one particular embodiment of the optical comparator, the optical thresholder is a saturable absorber that receives an optical control beam. If the saturable absorber saturates and turns transparent, the control beam passes through the thresholding device as an optical indicator beam. Photodetectors can be provided to convert the optical indicator beam to an electrical representation if desired. In an alternate embodiment, the optical thresholder is a semiconductor saturable absorber that is configured in a reverse-biased p-i-n layout, where the input beam generates electron/hole charge carriers in the saturable absorber that generate a current flow that can be measured as the indicator signal. When the semiconductor saturable absorber saturates, it turns transparent and the maximum number of charge carriers are generated.
For the optical signal attenuator the threshold level is varied and established by a control signal to selectively control the signal attenuation without saturating the optical thresholding device. In this embodiment, the intensity of the optical beam is less than that of the control signal.
Additional objects, advantages, and features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings.